This invention relates to the measurement of surface roughness of a material created by grinding, polishing, or machining of the surface, by the use of acoustic emission transducers and instrumentation.
A common practice is to attach a high frequency piezoelectric transducer to an object to be measured and record the RMS level of the signal produced by the particular process i.e., grinding, polishing or machining. The amplitude of the RMS signal is the primary measurement tool for correlating the AE signal to a particular parameter created by the process. Such acoustic emission techniques are also used for head disk interference (HDI) measurements by manufacturers of hard disk media for computers. However, major problems are encountered with these techniques. The measurements are very sensitive to the relative velocity between two contacting surfaces of a given roughness, and are effected by the transducer sensitivity and instrument gain.
Apparatus constructed and methods performed in accordance with the preferred embodiment of this invention have significant advantages. In particular, the accuracy measurements of a surface condition are not adversely effected by velocity effects and instrument gain. The preferred embodiment of the invention uses the ratio between two signals which are determined by averaging two signals that respectively correspond to two frequency components of the signal produced by the transducer to provide a measurement of surface roughness that is substantially insensitive to the amplitude of the transducer output signal.